A typical power supply circuit for driving LEDs receives power from an AC source. During a normal mode of operation, the power supply circuit generates either a constant voltage or a constant current to a DC load. The power supply circuit includes a first-stage power factor correction circuit that operates to maintain the current drawn from the AC source substantially in phase with the voltage of the AC source. The power factor correction circuit provides a first-stage output voltage to a second-stage DC-to-DC converter circuit. The second-stage DC-to-DC converter circuit generates the constant voltage or the constant current for the DC load.
The second-stage DC-to-DC converter can be configured as a resonant type DC-to-DC converter. A typical resonant type DC-to-DC converter can include a gate driver integrated circuit (IC) that generates a first gate driver signal to a first metal oxide semiconductor transistor (MOSFET) and generates a second gate driver signal to a second MOSFET. The two MOSFETs are connected in a half-bridge configuration. The two MOSFETS drive the primary winding of an output transformer via a resonant inductor. A resonant capacitor is connected across the primary winding. A center-tapped secondary winding has the center tap connected to a secondary ground reference. A first output terminal of the secondary winding is connected to a first diode. A second output terminal of the secondary winding is connected to a second diode. The two diodes are configured as half-bridge rectifier that drives a DC load, such as a plurality of light-emitting diodes (LEDs). An output filter capacitor is connected across the DC load. A current sense resistor is connected between the DC load and the secondary ground reference and generates a feedback voltage proportional to the current through the DC load. The feedback voltage is connected to a microcontroller, which also receives a dimmer control signal from a control source. The microcontroller compares the feedback voltage to the dimmer control signal and adjusts a frequency control signal to the gate driver IC to cause the gate driver IC to adjust the frequency of the first and second gate driver signals to thereby control the current provided to the DC load.
An LED driver should be sufficiently reliable to survive abnormal transient conditions, such as a sudden load removal, a shorted load, start into an open load, or the like, so that the LED driver resumes normal operation after the abnormal condition is no longer present. The above-described typical LED driver with the resonant type half-bridge converter has a drawback that hard-switching can occur during an abnormal transient condition. In a resonant type half-bridge converter, the two MOSFETs are switched such that the transitions between the on states and the off states occur at low (or zero) current levels when the circuit is operating normally. During an abnormal transient condition, hard-switching can occur such that the transitions between the on states and the off states of the MOSFETs occur at higher current levels. Hard-switching results in increased wasted power during the switching transitions and can result the failure of one or both MOSFETs and failure of the overall LED driver.